The invention relates in general to video signal processing and in particular to a method and device by which the phase position of a sampling clock relative to an analog signal to be sampled can be reconstructed and controlled.
When analog signals, for example, analog video signals are processed or evaluated, these signals are often sampled in coupled form. Specifically in the case of analog video signals, line-coupled sampling is often implemented. This means that the analog signal is sampled at the sampling frequency (also called the pixel frequency) of the pixels to be displayed on the screen or monitor. A single pixel is typically sampled during a single sample period.
During the coupled sampling of analog signals, not only is the sampling or pixel frequency relevant, but also the phase position of the sampling clock relative to the analog signal to be sampled, for example, a pixel signal. For this reason, synchronization of the phase position of the sampling clock with the analog signal is generally desirable. In addition, it is desirable that the phase position of the sampling clock with respect to the analog signal be kept relatively constant.
If an unfavorable phase position occurs, as illustrated in the graph of FIG. 1a, then inaccurate sampling of the analog signal results. For example, the curve of an analog signal 10 illustrated in FIG. 1a carries the relevant information at alternating peak signal values W1 and W2. In video applications, for example, a black pixel could be associated with the peak signal value W1, while a white pixel could be associated with the peak signal value W2. In the example of FIG. 1a, the analog signal 10 is sampled during each of the time intervals Ta at corresponding sampling instants 12, at which the sampling values of the analog signal 10 illustrated by the points 14 are obtained. The time interval Ta represents the inverse of the sampling frequency.
For the phase position illustrated in the graph of FIG. 1a, the same sampling value 14 for each sampling instant 12 is obtained. A number of sampling results E are obtained which cannot be accurately associated with either of the peak sampling values W1 or W2. Relating this to the black and white pixel example, since the sampling results E lie in a straight line midway between the two signal values W1 and W2, a gray value results for each sampling value 14 which does not represent either a black or a white pixel. This situation is also illustrated in FIG. 1a by the sampling distance D being equal between W2 and E and between W1 and E. That is, there is zero separation distance between the sampling results E in FIG. 1a. As a result, any meaningful separation and interpretation of the peak sampling values W1 and W2 from the sampling results E are difficult to achieve.
Conversely, the graph in FIG. 1b illustrates the situation where the phase position for the sampling clock relative to the analog signal 10 results in the sampling values 14 differing by the highest possible distance D between the sampling results E1, E2. In other words, the analog signal 10 is sampled each time 14 at the peak values of W1 and W2. This yields both the sampling results E1, which are associated with the peak sampling value W1, and the sampling results E2, which are associated with the peak sampling value W2. Thus, as seen by the examples of FIGS. 1a and 1b, the phase position of the sampling clock relative to the analog signal 10 (i.e., the synchronization therebetween) is relevant as to the ability to properly evaluate and interpret the analog signal 10.
During analog video signal sampling, the required synchronization is often implemented by synchronizing a phase-locked loop (PLL), to control the phase position of the sampling clock, to a time synchronization pulse, also called an H-Sync, which pulse is normally transmitted in the analog video signals. However, in the case of computer video signals, the situation often arises whereby, for example, the pixels are shifted in phase relative to the H-Sync. As a result, additional phase correction of the sampling clock is required.
In the example above, however, as well as in general, the absolute value of the phase deviation of the analog signal from the sampling clock, or if present, the synchronization pulse, is in general not known since only one sampling point per to-be-sampled signal is available. Specifically, in the case of line-coupled sampling of an analog video signal, only one sampling value per pixel is normally available.
What is needed is a method and device by which the phase position of the sampling clock relative to the to-be-sampled signal can be simply and reliably determined and controlled.